The invention relates to a method for tempering preforms made of a thermoplastic material wherein the preforms are provided for blow deformation into containers, in which the preform is provided along the periphery with a temperature profile which is produced in that striplike regions or sectors extending in the direction of a longitudinal axis of the preform are heated differentially.
In addition, the invention relates to a device for tempering preforms made of a thermoplastic material which has at least one conveyor line for the preforms and at least one heating means and provides the preforms with a temperature profile in the peripheral direction of the preforms.
Such a method is employed when, for example, containers whose cross section differs from a round shape are to be produced. The difference, for example, may consist in producing containers with an oval cross section or for example with a triangular or rectangular cross section.
The production of such unround containers has already been described in U.S. Pat. No. 3,775,524. First, symmetrical tempering of the preforms takes place; then, the temperature is selectively increased in selected regions. Additional variants for producing temperature profilings in the peripheral direction of preforms are likewise described in U.S. Pat. No. 3,632,713, U.S. Pat. No. 3,950,459 and U.S. Pat. No. 3,892,830. Temperature conditioning by selective shading is indicated in DE-OS 3,314,106.
U.S. Pat. No. 5,292,243 discloses that two preforms may be simultaneously subjected to temperature conditioning in the peripheral direction. A combination of methods known in the prior art for the temperature conditioning of preforms is found in EP-OS 0,620,099.
Common to the previously known methods is that either there is a relatively high cost for apparatus or only relatively poor reproducibility can be ensured.
The object of the present invention is therefore to indicate a method of the type mentioned at the beginning by which high quality temperature conditioning can be obtained at low cost.
This object is accomplished, according to the invention, in that stepwise tempering is carried out for successively timed thermal conditioning of unlike regions of the preform.
An additional object of the present invention is to construct a device of the type mentioned at the beginning so as to permit selective tempering of the preform with high reproducibility.
According to the invention, this object is accomplished in that a rotational drive having a steplike mode of operation is provided for performing a motion of rotation of the preform.
Stepwise tempering of the preform makes it possible to arrange, for example, along a transport path of the preform, conventional radiant heaters of the prior art with IR radiators and to expose various regions of the preform to radiation for various lengths of time. In this connection in particular, no troublesome coordination between the rate of the longitudinal motion of the preform in the direction of transport and the rate of rotation is required.
Implementation of the method using simple equipment may be effected in that stepwise rotation is carried out with phases of motion and rest.
The drive method may likewise be simplified in that one and the same direction of rotation is provided for stepwise rotation.
To prevent unsymmetries in heating, it is alternatively possible for alternating directions of rotation to be provided.
A very symmetrical temperature profile in the peripheral direction can be obtained in that a change in the direction of rotation is made each time that a complete rotation of 360.degree. is carried out.
Depending upon the respective application requirements, it is alternatively possible for a change in the direction of rotation to be made each time that an integral multiple of a complete rotation of 360.degree. is carried out.
For containers with the mouth section arranged symmetrically with regard to the container center line, it is advantageous for a symmetrical temperature profile to be produced in the peripheral direction.
In the case of a mouth section arranged unsymmetrically, it may be advantageous for an unsymmetrical temperature profile to be produced in the peripheral direction.
Uniform distribution of material in blow deformation of the container is promoted in that a stretch ratio in the region from 1:1.5 to 1:2.2 is set. An upper interval limit of 2.0 is preferable.
Adaptation of the stretch ratio to the shape of the container may be effected in that with increasing unroundness of the containers to be produced the stretch ratio moves toward 1:1.5.
An additional positive influence on the distribution of material may be obtained in that the stretch operation is carried out with the aid of compressed air.
It is likewise advantageous for the distribution of material that the supply of compressed air be effected so that, during stretching, contact of the preform with the blow mold is prevented during stretching and contact of the expanding preform takes place exclusively in the region of the inner wall of a blow mold extending in the vicinity of a minimum mold diameter with regard to an unround container cross section.
Optimal coordination of stretching rate and compressed air supply is effected in that the supply of compressed air during performance of the stretching operation is selected so that contact in the longitudinal direction of the expanding preform with the blow mold takes place exclusively in a center region, in the longitudinal direction of the mold.
Another variant for accomplishing step operation consists in that the preforms are moved at a constant rate and the heating means are controlled stepwise.
It is likewise possible for an alternating facing arrangement of heating means along the direction of transport of the preforms to be provided and for the preforms to be carried past the heating means nonrotating and heated stepwise in unlike regions of their extension. A facing arrangement may also be selected for reasons of space.
An especially simple arrangement is obtained in that the preforms are first uniformly tempered and then blown on by a cooling nozzle for partial cooling.